An electric eel attacking a pretend alligator K.C. Catania/Vanderbilt University
An electric eel attacking a pretend alligator
K.C. Catania/Vanderbilt University
Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world
Species: the electric eel, Electrophorus electricus. Despite its name, it’s a knifefish not an eel
Habitat: the waters of South America
More than 200 years ago naturalist Alexander von Humboldt recounted seeing electric eels leaping out of the water to attack horses in the Amazon. The locals herded some 30 horses and mules into a small pool provoking the eels to attack – and kill some of the – horses.
The method, he wrote, was used to “fish with horses”, because locals could pick up the exhausted eels safely after the mayhem. But it was thought to be an exaggeration because nobody else had witnessed a similar assault.
Until now, that is. They’ve been filmed leaping out of the water and delivering massive shocks to powerful predators.
Kenneth Catania from Vanderbilt University in Nashville, Tennessee, saw the jumps when he used a metal-rimmed net to transfer eels in his lab. “I was definitely surprised,” he says. “This isn’t something electric eels typically do.”
Electric eels emit electric pulses in water, where they have the choice of two different settings. Using three electric organs that fire at the same time, they release a low voltage to sense their environment but can switch to high power to stun prey or defend themselves.
The electrical organs extend from the rear of the fish’s body up most of its length, leaving a little room in the front for the stomach and other organs. “They are like giant flashlights with a lot of battery packed into their back end,” says Catania.
When the eels lunged at Catania’s net, he initially thought they were simply trying to avoid it. “Sometimes up to half of their body rises out of the water,” he says. “They don’t seem like dexterous animals but they are good at it.”
However he noticed that the eels would deliver high-voltage blasts while keeping their chin in contact with the net during a leap. So he set up an experiment to record the electric pulses by placing a conductive rod in an aquarium.
He then dunked a fake alligator head laced with LEDs into a tank, which would light up if the eels attacked (see video below).
When the eels jumped onto a mock predator, the current it received increased as the eels slithered higher up. Slowing down the videos revealed that the tip of their electric organ always touched their target during a leap.
Off or on
The eels have only a single high-voltage setting, so can’t tweak its power output so instead it seems to be using a trick: delivering the shock directly to a threat, instead of distributing it through water.
The recordings show both voltage and current increasing the higher up the eel slithered.
“It seems clear that the eels are actively keeping contact with their chin to try to target the object they see as a threat,” says Catania.
Catania has previously found that the eels have complex hunting tactics. Emitting a specific pattern of electric pulses can uncover hidden, motionless prey by making muscles twitch involuntarily. Another type of shock hijacks the muscles and blasts the prey to death. The targeted leaping is another example of their sophistication.
“It is a beautiful example of how the eel has evolved a fairly simple behaviour that exploits the basic physics of electricity,” says Bruce Carlson from Washington University in St Louis, who studies how electric fish use sensory information.
Catania thinks the behaviour is an evolved adaption to life in the Amazon, where water retreats during the dry season and leaves eels trapped in small bodies of water and exposed to predators.
Journal reference: PNAS, DOI: 10.1073/pnas.1604009113